DIODES AND THEIR APPLICATION
We continue to study semiconductor devices. I want to pay closer attention to them because their importance in radio electronics can hardly be overestimated. In this lesson, a simple practical assignment will be offered to consolidate the material. In all other respects, the significance of this lesson is no different from the previous ones. If you have noticed, in all lessons, I try to lay out the main ideas on the topic so as not to overload young radio amateurs with incomprehensible mathematical calculations, etc., except for detailed explanations when necessary. And so, as in the previous lessons, what is highlighted in the important blocks must be memorized. Let's begin!
Semiconductors and their properties
Today, the diode family includes dozens of semiconductor devices bearing the name diode. Here we will discuss only some of the devices you will primarily have to deal with. Schematically, a diode can be represented as two semiconductor plates, one of which has p-type conductivity, and the other — n-type. In (Fig. 1, a), the holes prevailing in the p-type plate are conventionally represented by circles, and the electrons prevailing in the n-type plate by black spheres of the same size. These two regions are the two electrodes of the diode: the anode and the cathode.
The anode, i.e., the positive electrode, is the p-type region, and the cathode, i.e., the negative electrode, is the n-type region.
Contact metal layers are applied to the outer surfaces of the plates, to which the wire leads of the diode electrodes are soldered. Such a semiconductor device can be in one of two states: open, when it conducts current well, and closed, when it conducts current poorly.
If a direct current (DC) source, for example, a galvanic cell, is connected to its electrodes so that its positive pole is connected to the diode's anode (i.e., the p-type region) and the negative pole to the cathode (i.e., the n-type region) (Fig. 1, b), the diode will be in the open state, and a current will flow in the formed circuit, the value of which depends on the applied voltage and the properties of the diode.
With this polarity of battery connection, electrons in the n-type region move from minus to plus, i.e., towards the p-type region, and holes in the p-type region move towards the electrons — from plus to minus. Meeting at the boundary of the regions, called the electron-hole junction or, in short, the p-n junction, electrons seem to jump into the holes; as a result, upon meeting, both cease to exist. The metal contact connected to the negative pole of the cell can give the n-type region a virtually unlimited number of electrons, replenishing the lack of electrons in this region, and the contact connected to the positive pole of the cell can accept the same number of electrons from the p-type region, which is equivalent to introducing a corresponding number of holes into it.
In this case, the resistance of the p-n junction is small, resulting in a current flowing through the diode called the forward current. The larger the area of the p-n junction and the voltage of the power source, the greater this forward current. If the poles of the cell are swapped, as shown in (Fig. 1, c), the diode will be in a closed state. In this case, the electrical charges on the diode will behave differently. Now, moving away from the p-n junction, electrons in the n-type region will move to the positive contact, and holes in the p-type region — to the negative contact of the diode. As a result, the boundary of regions with different types of electrical conductivity will expand, forming a zone depleted of electrons and holes (in Fig. 1, c, it is shaded), which, consequently, offers very high resistance to the current. However, a small exchange of current carriers between the diode regions will still occur in this zone. Therefore, a current will flow through the diode, but many times smaller than the forward current. This current is called the reverse current of the diode.
On graphs characterizing the operation of a diode, the forward current is denoted as I_f (forward), and the reverse current as I_r (reverse). What if a diode is included in an alternating current (AC) circuit? It will open during positive half-cycles at the anode, freely passing current in one direction — forward current I_f, and close during negative half-cycles at the anode, almost blocking current in the opposite direction — reverse current I_r.
These properties of diodes are used in rectifiers for converting alternating current into direct current.
The voltage at which the diode opens and forward current flows through it is called the forward voltage (U_f), and the voltage of reverse polarity at which the diode closes and reverse current flows through it is called the reverse voltage (U_r).
At forward voltage, the resistance of a good quality diode does not exceed a few tens of ohms; at reverse voltage, its resistance reaches tens, hundreds of kilohms, and even megohms.
This is easy to verify if the reverse resistance of the diode is measured with an ohmmeter. The internal resistance of an open diode is not a constant value and depends on the forward voltage applied to the diode: the greater this voltage, the greater the forward current through the diode, and the lower its forward resistance.
The resistance of a diode can be judged by the voltage drop across it and the current through it.
For example, if a forward current I_f = 100 mA (0.1 A) flows through the diode, and at the same time a voltage of 1 V drops across it, then (according to Ohm's law) the forward resistance of the diode will be: R = 1 / 0.1 = 10 Ohms. In the closed state, almost all the voltage applied to the diode drops across it, the reverse current through it is extremely small, and the resistance, therefore, is large.
The dependence of the current through the diode on the value and polarity of the voltage applied to it is depicted as a curve called the volt-ampere characteristic (V-I characteristic) of the diode.
You can see such a characteristic in (Fig. 2). Here, the values of the forward current I_f are plotted upwards along the vertical axis, and the reverse current I_r downwards. The values of the forward voltage U_f are indicated to the right along the horizontal axis, and the reverse voltage to the left. On such a V-I characteristic, one distinguishes the forward branch (in the upper right part), corresponding to the forward current through the diode, and the reverse branch of the V-I characteristic, corresponding to the reverse current. It shows that the I_f current of the diode is hundreds of times greater than the I_r current. For example, even at a forward voltage U_f = 0.5 V, the current I_f is 50 mA (point 'a' on the characteristic), at U_f = 1 V it increases to 150 mA (point 'b'), and at a reverse voltage U_r = 100 V, the reverse current I_r does not exceed 0.5 mA (500 μA). Calculate how many times the forward current is greater than the reverse current at the same forward and reverse voltage.
The forward branch goes steeply upwards, as if pressing against the vertical axis. It characterizes the rapid growth of forward current through the diode with an increase in forward voltage. The reverse branch, as you can see, runs almost parallel to the horizontal axis, characterizing the slow growth of the reverse current. The presence of noticeable reverse current is a disadvantage of diodes. Almost all germanium diodes have such V-I characteristics. The V-I characteristics of silicon diodes are shifted slightly to the right.
This is explained by the fact that a germanium diode opens and begins to conduct current at a forward voltage of 0.1-0.2 V, and a silicon one at 0.5-0.6 V.
The device, using the example of which I told you about the properties of a diode, consisted of two plates of semiconductors of different electrical conductivities connected by flat planes. Such diodes are called junction diodes. In reality, a junction diode is a single semiconductor plate in the volume of which two regions of different electrical conductivities are created. The technology for manufacturing such diodes is as follows. On the surface of a square plate with an area of 2-4 square mm and a thickness of several fractions of a millimeter, cut from a semiconductor crystal with electronic conductivity, a small piece of indium is melted. Indium fuses firmly with the plate. During this process, indium atoms penetrate (diffuse) into the depth of the plate, forming a region with a predominance of hole conductivity in it (Fig. 3, a). The result is a semiconductor device with two regions of different types of electrical conductivity, and a p-n junction between them.
An indium droplet and a metal disk or rod with lead wires serve as the contacts of the diode electrodes. This is how the most common junction germanium and silicon diodes are constructed. The appearance of some of them is shown in (Fig. 3, b). The devices are enclosed in all-metal or glass cases with glass insulators, which allows them to be used for work in high humidity conditions. Diodes designed for significant forward currents have screws with nuts for mounting them on mounting panels or chassis of electronic devices.
Junction diodes are marked with letters and numbers, for example: D226A, D242. The letter D in the device marking means "diode", the numbers following it are the factory serial number of the design. The letters at the end of the diode designation indicate varieties of device groups.
Junction diodes are primarily designed to work in AC rectifiers of power supply units for radio equipment, which is why they are also called rectifier diodes.
Now let's get acquainted with the principle of converting alternating current into direct current. You can see the diagram of the simplest AC rectifier in (Fig. 4, a). The AC voltage of the lighting network is applied to the input of the rectifier. A resistor R_load is connected to the output of the rectifier, symbolizing the load powered by the rectifier. The function of the rectifying element is performed by diode V. The essence of the operation of such a rectifier is illustrated by the graphs placed in the same figure. During positive half-cycles of the voltage at the anode, the diode opens. At these moments, the forward current of the diode I_f flows through the diode, and therefore through the load connected to the rectifier. During negative half-cycles of the voltage at the anode, the diode closes, and an insignificant reverse current of the diode I_r flows in the entire circuit in which it is included.
The diode seems to cut off most of the negative half-waves of the alternating current (in Fig. 4, a, it is shown by dashed lines). And here is the result: a current flows through the load R_load, connected to the network through diode V, which is no longer alternating, but a pulsating current — a current of one direction, but changing in value with a frequency of 50 Hz. This is the form of rectified alternating current. Thus, the diode is a device with a pronounced unilateral conductivity of electric current. And if we neglect the small reverse current (which is done in practice), which in working diodes does not exceed small fractions of a milliampere, we can assume that the diode is a unilateral current conductor.
Is it possible to power a load with such a current? Yes, it is rectified after all. But not every load. An incandescent lamp, for example, can be, provided, of course, that the output voltage does not exceed the voltage for which the lamp is designed. Its filament will be heated not constantly, but by pulses following with a frequency of 50 Hz. Due to thermal inertia, the filament will not have time to cool down in the intervals between pulses, so the flickering of the light will be barely noticeable.
But a receiver cannot be powered by such a current. Because in its amplifier circuits, the current will also pulsate with the same frequency. As a result, a low-tone hum with a frequency of 50 Hz, called AC hum, will be heard in the headphones or the loudspeaker head at the receiver output.
This disadvantage can be partially eliminated if a filtering electrolytic capacitor (C_f) of large capacity is connected in parallel with the load at the rectifier output; this is shown in (Fig. 4, b). Being charged by the current pulses, the capacitor (C_f) discharges through the load R_load at the moment the current drops or disappears (between pulses). If the capacitor is of sufficiently large capacity, then during the time between current pulses it will not have time to discharge completely, and a current will be continuously maintained in the load. The current maintained by charging the capacitor is shown in (Fig. 4, b) by a solid wavy line.
But even with such a somewhat smoothed current, you still cannot power a receiver or amplifier: it will hum, since the pulsations are still very noticeable. In the rectifier, whose operation we are now analyzing, the energy of only half of the AC waves is usefully utilized. Such rectification of alternating current is called half-wave, and the rectifiers are called half-wave rectifiers.
However, rectifiers built according to such schemes have two significant drawbacks. The first one is that the rectified current voltage is approximately equal to the mains voltage, while a lower voltage is needed to power transistor designs, and often a higher voltage for tube ones. The second drawback is the inadmissibility of connecting a ground to a receiver powered by such a rectifier. If the receiver is grounded, the current from the mains will go through the receiver to the ground — the fuses may blow. In addition, a receiver or amplifier powered by such a rectifier, and thus having direct contact with the mains, is dangerous — you can get an electric shock.
Both of these drawbacks are eliminated in a rectifier with a transformer (Fig. 5). Here, it is not the mains voltage that is rectified, but the voltage of the secondary (II) winding of the mains transformer T. Since this winding is isolated from the primary mains winding I, the electronic device has no contact with the mains, and a ground can be connected to it. In the rectifier in (Fig. 5), there are four diodes connected in a so-called bridge circuit. The diodes are the arms of the rectifier bridge. The load R_load is connected to diagonal 1-2 of the bridge. In such a rectifier, during each half-cycle, two diodes of opposite arms of the bridge operate alternately, connected in series with each other, but opposite to the second pair of diodes.
Try to grasp and remember the classic diode bridge circuit!
When there is a positive voltage half-cycle on the upper (according to the diagram) terminal of the secondary winding, the current flows through diode V2, load R_load, diode V3 to the lower terminal of winding II (graph a). Diodes V1 and V4 are closed at this time.
During the other half-cycle of AC voltage, when the plus is on the lower terminal of winding II, the current flows through diode V4, load R_load, diode V1 to the upper terminal of the winding (graph b). At this time, diodes V2 and V3 are closed and, naturally, do not pass current through themselves.
And here are the results: the voltage signs on the terminals of the transformer's secondary winding change, but a current of one direction flows through the rectifier load (graph c). In such a rectifier, both AC half-cycles are usefully utilized, so such rectifiers are called full-wave rectifiers.
The efficiency of a full-wave rectifier compared to a half-wave one is obvious: the pulsation frequency of the rectified current has doubled, the dips between pulses have decreased. The average DC voltage at the output of such a rectifier is approximately equal to the AC voltage acting across the entire secondary winding of the transformer. And if the rectifier is supplemented with a filter that smooths out the ripples of the rectified current, the output voltage will increase by 1.4 times, i.e., by about 40%. This is exactly the rectifier I will later recommend to you for powering transistor designs.
Now about the point-contact diode. The appearance of one of such devices and its structure (significantly enlarged) are shown in (Fig. 6). This is a D9 series diode. The letter D in its marking means diode, and the number 9 is the factory serial number of the design. The rectifying element of the diode is a thin and very small (area about 1 square mm) plate of a germanium or silicon semiconductor of type n and a tungsten wire resting with a sharp end on the plate. They are soldered to pieces of silvered wire about 50 mm long each, which are the diode leads. The entire structure is inside a glass tube with a diameter of about 3 mm and a length of less than 10 mm, sealed at the ends.
After assembly, the diode is formed — a current of a certain value is passed through the contact between the semiconductor plate and the tip of the tungsten wire. At the same time, a small region with hole conductivity is formed under the tip of the wire in the semiconductor crystal. The result is an electron-hole junction having a unilateral current conductivity. The semiconductor plate is the cathode, and the tungsten wire is the anode of the point-contact diode.
The anode lead of D9 series diodes is indicated by color marks on their bodies. The electrodes of a D2 series point-contact diode are designated by a diode symbol on one of its ribbon leads. In a point-contact diode, the contact area of the wire tip with the surface of the semiconductor plate is extremely small — no more than 50 μm. Therefore, the currents that point-contact diodes can rectify for a long time are small. Radio amateurs use point-contact diodes mainly for detecting high-frequency modulated oscillations, so they are often called high-frequency diodes.
For both junction and point-contact diodes, there are maximum permissible values of forward and reverse currents, depending on the forward and reverse voltages, which determine their rectifying properties and electrical strength. These are their main parameters. The junction diode D226V, for example, can rectify a current up to 300 mA for a long time. But if it is included in a circuit consuming a current of more than 300 mA, it will heat up, which will inevitably lead to thermal breakdown of the p-n junction and failure of the diode. The diode will also be broken down if it turns out to be in a circuit where a reverse voltage of more than 400 V is applied to it. The permissible rectified current for a D9A point-contact diode is 65 mA, and the permissible reverse voltage is 10 V.
The main parameters of semiconductor diodes are indicated in their datasheets and reference tables. Exceeding the limit values leads to failure of the devices. The main parameters of the most common point-contact and junction semiconductor diodes can be found here.
ZENER DIODE AND ITS APPLICATION
A Zener diode is also a diode, but it is not intended for rectifying alternating current, although it can perform such a function, but for stabilization, i.e., maintaining constant voltage in the power supply circuits of electronic equipment.
The appearance of one of the designs of Zener diodes most common among radio amateurs and its graphical symbol are shown in (Fig. 8). By design and principle of operation, widely used silicon Zener diodes are similar to junction rectifier diodes. But the Zener diode does not operate on the forward section of the V-I characteristic, like rectifier or high-frequency diodes, but on the reverse branch of the V-I characteristic, where a slight reverse voltage causes a significant increase in reverse current through the device.
To understand the essence of the Zener diode's operation, its V-I characteristic, shown in (Fig. 9, a), will help you. Here (as in Fig. 2), the reverse voltage U_r is plotted horizontally at a certain scale, and the reverse current I_r downwards vertically.
The voltage is applied to the Zener diode in reverse polarity, i.e., it is connected so that its anode is connected to the negative pole of the power source.
With such a connection, a reverse current I_r flows through the Zener diode. As the reverse voltage increases, the reverse current grows very slowly — the characteristic runs almost parallel to the U_r axis. But at a certain voltage U_r (in Fig. 9, a — about 8 V), the p-n junction of the Zener diode breaks down and a significant reverse current begins to flow through it. Now the V-I characteristic turns sharply and goes down almost parallel to the I_r axis. This section is the working one for the Zener diode. The breakdown of the p-n junction does not lead to damage to the device if the current through it does not exceed a certain permissible value.
In (Fig. 9, b), a diagram of a possible practical application of a Zener diode is given. This is a so-called parametric voltage regulator. With this connection, a reverse current I_r, created by the power source, whose voltage can vary within significant limits, flows through the Zener diode V. Under the influence of this voltage, the current I_r flowing through the Zener diode also changes, but the voltage across it, and therefore across the load R_load connected to it, remains practically unchanged — stable. The resistor R limits the maximum allowable current flowing through the Zener diode. You will have to deal with voltage regulators repeatedly in practice.
Here are the most important parameters of a Zener diode: stabilization voltage V_z, stabilization current I_z, minimum stabilization current I_z.min, and maximum stabilization current I_z.max.
The parameter V_z is the voltage that is created between the terminals of the regulator in operating mode. Our industry produces silicon Zener diodes for stabilization voltages from a few volts to 180 V.
The minimum stabilization current I_z.min is the smallest current through the device at which stable operation in the breakdown mode begins (in Fig. 9, a — dashed line I_z.min); with a decrease in this current, the device stops stabilizing the voltage.
The maximum allowable stabilization current I_z.max is the highest current through the device (do not confuse it with the current flowing in the circuit powered by the voltage regulator) at which the temperature of its p-n junction does not exceed the permissible one (in Fig. 9, a — dashed line I_z.max). Exceeding the current I_z.max leads to thermal breakdown of the p-n junction and, naturally, to device failure.
PRACTICAL WORK: Experiment with a diode
For a better understanding of the material of this lesson and to better fix in your memory your understanding of the properties of diodes, I suggest conducting this experiment. In an electrical circuit composed of a 3R12 battery (commonly called a flat battery) or a 9V battery, an incandescent bulb designed for a voltage of 3.5 V (or 6.3 V if it's a 9V battery) and a filament current of 0.28 A, include any diode from the D7, D226, KD226, KD220 series, etc., so that the anode of the diode is connected directly or through the bulb to the positive terminal of the battery, and the cathode to the negative terminal (Fig. a).
The bulb should burn almost as if there were no diode in the circuit. Reverse the order of connecting the diode electrodes in the circuit (Fig. b). Now the bulb should not burn. And if it burns, then the diode turned out to have a broken p-n junction. You can break such a diode to see how it is constructed — it is unsuitable for work as a rectifier anyway. But I hope the diode was good and the experiment was successful.
Why did the bulb burn during the first connection of the diode in the circuit, and not burn during the second? In the first case, the diode was open, since a forward voltage U_f was applied to it, the resistance of the diode was small, and a forward current I_f flowed through it, the value of which was determined by the circuit load — the bulb. In the second case, the diode was closed, since a reverse voltage U_r equal to the battery voltage was applied to it. The resistance of the diode was very large, and only an insignificant reverse current I_r flowed in the circuit, which could not heat the bulb filament.
In this experiment, the bulb performed a twofold function. First, it was an indicator of the presence of current in the circuit, and second, it limited the current in the circuit to 0.28 A and thus protected the diode from overload.
Moving on to the next lesson!
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